Two Keyboard Maestro scripts that start and stop time in a calendar named “Work”. I use this instead of a separate app and I’m fairly happy, because I don’t need to complicate my life and to bill clients. (Although you could also do the last step by writing some additional scripts that analyze the calendar).

Use the following keyboard shortcuts:

• ⌃⇧Home: Start a new task. The script asks for the name, project and notes for the current task. You can edit the projects in the script. Use a pipe (|) to separate projects and you’ll get a nice popup menu.
• ⌃⇧End: Stop the current task. Enjoy some amazing messages.

You can change the calendar in both AppleScript steps that are located at the bottom of the scripts if you aren’t happy with “Work”.